The simplest instance (FIG. 1a) of an electronic capacitor 10 includes a dielectric material 12 sandwiched between two metal electrodes 14, 16. More complex examples (not shown) include multi-layer devices which incorporate additional alternating layers of dielectric and electrode, electrically connected in parallel by terminations so as to cause the net capacitance value of the assembly to be equal to the sum of the values of the individual layers.
The dielectric 12 may be any electrical insulator, but required properties of the device 10 dictate which materials are satisfactory for the application at hand. One of the important properties of dielectric materials is dielectric constant (K). This property, along with the thickness (t) of the dielectric layer 12, determines the magnitude of capacitance C achievable per unit of active area A of the dielectric/electrode sandwich. When face area A is fixed, capacitance is directly proportional to the dielectric constant K and inversely proportional to dielectric thickness t. Commonly used dielectrics include mica, thin films of various oxides such as those of aluminum, tantalum and silicon, and various electronic ceramics whose dielectric constants span the range of about 6 to 20,000.
In general, it is desirable to get the highest capacitance per unit area from a given chip because of limited circuit board space, particularly in today""s miniature equipment such as wireless phones and hand-held computing devices. Accordingly, this high capacitance density will be attained by choosing a material with high dielectric constant and minimum thickness. Choice of dielectric is limited by losses and temperature stability requirements so thickness becomes a critical factor.
Ceramics are one of the most useful classes of dielectric materials for present applications. However, due to their fragility and the difficulty in firing them at thicknesses much below 0.004 inch, ceramics are usually used in multi-layer form, which yields high capacitance per unit board area but also causes high inductance, an undesirable property, relative to single layer devices. This high inductance drawback also applies to devices with a single dielectric layer and one or more electrodes buried within a multi-layer structure in which electrical contact(s) to said buried layer(s) are brought to the surface through vias, edge connections, etc. The present invention avoids any of these inductance-increasing methods and in fact makes practical a classic single layer ceramic capacitor with dielectric thickness of 0.001 inch or less.
Once a given dielectric is selected the size of the resulting device for any given capacitance is conventionally determined by the number of layers and their thicknesses. True single layer capacitors (SLCs) have lower inductance and thus higher resonant frequencies than their multi-layer counterparts and, as stated, are useful at higher frequencies, a necessity for many of today""s broadband, wireless and other applications. Since SLCs have a much more limited capacitance range than multi-layer devices, when area is predetermined; e.g., by a circuit configuration, it is very important to get the dielectric thickness as low as possible. True SLCs currently available are limited to about 0.004 inches in minimum thickness because of the fragility of the dielectric materials at lesser thickness.
This disclosure focuses on examples with thin ceramic dielectrics, which ceramics are by nature brittle and therefore very fragile in thicknesses below about 0.005 inch. The application discloses methods of manufacturing true SLCs with such ceramic dielectric having thickness as low as 0.0005 inch. The term xe2x80x9ctrue SLCxe2x80x9d is used to distinguish those physical embodiments and methods disclosed herein from single buried layer devices conventionally manufactured with the same design as multi-layer devices and having similar negative features.
What is needed is a physically and electrically reliable true single layer capacitor of high capacity per unit area using a thin ceramic dielectric that is less than 0.004 inch in thickness.
An SLC 10 (FIG. 1a) in accordance with the invention has a thin ceramic dielectric layer 12 (less than 0.004 inch and as low as 0.0004 inch or less), which has as electrodes conductive layers 14, 16 that are thick and strong enough either singly or together to give the structure 10 the required physical strength for manufacturing, handling and usage, and having electrical properties giving the device its required performance properties. An electrode(s) 14, 16, for example, is composed of (1) a ceramic-metal composite, (2) a porous ceramic infiltrated with metal or other conductive material, (3) a resin filled with metal or other conductive material, or (4) combinations of the above.
Thus, a very thin and in itself, fragile, dielectric layer 12 provides exceedingly high capacity per unit area, while thick, strong electrodes 14, 16 provide structural strength and support that protect the dielectric of the capacitor during manufacturing and usage.
Several exemplary methods are presented herein that are suitable for mass production of SLCs in accordance with the invention. Embodiments of the invention using an oxide of titanium as a very thin dielectric 12 are also disclosed.